The phr (parts per hundred parts of rubber by weight) data used in this specification are the conventional quantitative data used in the rubber industry for mixture formulations. The amount added in parts by weight of the individual substances here is always based on 100 parts by weight of the entire composition of all of the rubbers present in the mixture.
Pneumatic tires are strengthened by textile or metallic reinforcement, e.g., brass-coated steel cord, in order to withstand high mechanical stresses. Pneumatic tires comprise by way of example brass-coated steel cord in the belt, in the bead core, and optionally in the carcass. In order to ensure that the rubber-reinforcement composite is durable, the embedding rubber mixture (rubberizing mixture) is intended to exhibit good adhesion to the reinforcement, and this adhesion should not be impaired by aging and by storage in moist conditions. The vulcanizates should moreover exhibit high dynamic and mechanical strength and low susceptibility to cracking and to crack propagation.
The adhesion of rubber to textile reinforcement is achieved by way of impregnation (e.g. with resorcinol-formaldehyde resins in combination with rubber latices (RFL dip)) by the direct method using adhesive mixtures or by way of adhesive solutions of unvulcanized rubber using polyisocyanates.
The rubber-metal adhesion can be advantageously influenced by use of what are known as reinforcing resins in the rubberizing mixture. Examples of known reinforcing resins are lignin, polymer resins, and phenol-formaldehyde resins with hardener. A method that has long been known for improving the rubber-metal adhesion is to use cobalt salts and/or a resorcinol-formaldehyde-silica system, or a resorcinol-formaldehyde system as additions for the rubberizing mixtures. Rubberizing mixtures with cobalt salts and with a resorcinol-formaldehyde-silica system are known by way of example from KGK Kautschuk Gummi Kunststoffe No. 5/99, pp. 322 to 328, from GAK August 1995, p. 536, and from U.S. Pat. No. 7,307,116.
Fillers used in known rubberizing mixtures are carbon black and/or silica in the following carbon-black-to-silica ratios: from 100:0 to 80:20, or else from 20:80 to 0:100.
Sulfur-crosslinkable rubberizing mixtures are known from U.S. Pat. Nos. 5,871,597 and 6,169,137 (see comparative experiments). In those documents they are used by way of example as rubberizing mixtures for the belt. The carbon blacks used in U.S. Pat. No. 5,871,597 and U.S. Pat. No. 6,169,137 are those of type N326. Along with type N330, this is one of the typical carbon blacks for rubberizing mixtures.